Every year at this time, officers note a rise in the number of people who fall victim to jammers - crooks who swipe handbags and valuables left on passengers seats while the owners are stuck in traffic.

Now they are to mount a series of high visibility patrols in blackspot areas, usually on commuters making the slow crawl home at rush hour.

And officers will be opening the doors of cars with goods in plain view to give drivers crime prevention tips.

Sgt Andrew Lewis acknowledged the idea would surprise some but said it was crucial to drive home the dangers of leaving valuables on view.

Similar tactics have been used in the past by anti-burglary officers to warn people of the dangers of leaving their doors and windows unlocked, particularly during the hot summer nights.

"Opening people's car doors while they are stopped at the traffic lights is a bit of a shock tactic to make drivers stop and think that although this time it is a police officer opening their door, next time it could be a thief," he said.

The rise in jamming is seen at this time of year as thieves take advantage of the darker nights.

They usually pounce when people are parked at traffic lights, either opening the car door or smashing the window before snatching valuables such as handbags and lap-tops that can be sold on the black market.

Teams of police community support officers will be patrolling areas where traffic stops, like Belgrave Middle-way, Pershore Road, Bristol Road, Priory Road, Edgbaston Road, Hayden Way and Stratford Road.

Sgt Lewis advised drivers to keep all car doors locked, windows shut and valuables like handbags and lap-tops out of sight under a seat or in a glove box.